Series of parallel electrical conductors held together by interwoven braiding

ABSTRACT

A flat cable comprises a series of parallel electrical conductors (1 to 6) each carrying a spiral-wound tape insulation layer (30). The conductors are held together by braiding formed of a plurality of tapes (10 to 22) or fibres interwoven between the conductors. The braiding is preferably of a thermoplastic material which may be heat bonded to the insulation in order to set the cable in a desired shape e.g. for a wiring loom. The insulation and braiding is preferably formed from a mixture of polytetrafluoroethylene (PTFE) and a copolymer of tetrafluoroethylene and perfluoro(propylvinylether).

TECHNICAL FIELD

The present invention relates to a flat cable construction comprising aseries of parallel longitudinally extending electrical conductorsarranged side-by-side.

BACKGROUND OF THE INVENTION

Flat cables are well known and are used in a variety of applicationswhere multiple electrical connections are to be made between pieces ofelectrical equipment. The flat orientation of the conductors allowsindividual conductors to be readily identified when connections arebeing made or when end connectors are being attached. Conventionally,flat cables are made by extruding the insulating material onto theparallel conductors.

Conventional extrusion suffers from a number of disadvantages,particularly where high performance insulation is required, e.g. in thedefence industry. Firstly, extruded flat cables are very resistant tobending in a direction parallel to the plane of the cable. This makes itdifficult to form conventional flat cables into pre-configured wiringharnesses, and means that the flat cable must be routed in situ. Itwould be desirable to be able to produce a flat cable construction whichcould be bent in any direction, such as to allow the production of"drop-in" pre-configured wiring harnesses, particularly where it isdifficult to route cables in situ due to restricted access or spacelimitations.

A second disadvantage is that such extruded insulation material does notnecessarily give the best mechanical and electrical insulationproperties. Also in order to safeguard the integrity of the insulation,the thickness of extruded insulation tends to be greater than, forexample, that of tape-wound insulation. Insulating tapes can beprocessed, such as by pre-stretching in order to provide the desiredmechanical and electrical properties.

It would therefore be desirable to provide a flat cable constructionwhich mitigates some or all of these disadvantages.

U.S. Pat. No. 3 582 537 discloses a woven ribbon cable wherein a wovenlattice structure holds a plurality of insulated conductor wires in agiven spaced parallel relationship and has its warp and weft membersbonded to each other at the intersections thereof.

W091/17551 discloses an electrical insulating composite materialcomprising an intimate admixture of a thermoplastic copolymer oftetrafluoroethylene and perfluoro (propyl vinyl ether) and coagulateddispersion type polytetrafluoroethylene (PTFE), or of porous expandedPTFE. Tape made from the composite material may be wrapped around aconductor and sintered to fuse the overlapping areas of the tapetogether.

U.S. Pat. No. 3 654 381 discloses a flat cable woven with a warpconsisting solely of conductors. The weft threads are woven directlyinto the conductors to form the final woven cable.

SUMMARY OF THE INVENTION

The present invention provides a flat cable which comprises a series ofparallel longitudinally extending electrical conductors arrangedside-by-side, each conductor having an electrically insulating covering,the parallel conductors being held together by braiding comprising atleast one filament interwoven between the conductors, wherein thebraiding is thermoplastic and is heat-bonded to the insulating coveringon the conductors.

As used herein, the term "conductor" relates to a single conductor orwire or a group of two or more conductors or wires twisted together.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts one conductor with spirally wound insulating tape aroundit.

FIG. 2 depicts insulated flat parallel conductors with braidinginterwoven between the conductors.

Whilst the electrically insulating covering can be extruded onto eachconductor, where thin high performance insulations are required it ispreferred, as shown in FIG. 2, to wrap one or more layers of aninsulating tape 30 in a spiral manner around the conductor 1.Preferably, the spirally wound turns of tape overlap by up to 50% of thewidth of the tape in order to maintain integrity of the insulation.Typically, from 1 to 5, usually 2 or 3 layers of tape are applied. Thetape is preferably formed from a mixture of polytetrafluoroethylene(PTFE) and PFA copolymer of tetrafluoroethylene (TFE) andperfluoro(propylvinylether) (PPVE) as disclosed in our internationalapplication PCT/GB91/00661. In particular, it is preferred that theinsulation shall be formed of alternating layers of porous expandedmaterial and non-porous material. Such tape wrap constructions show goodabrasion and cut-through resistance and good electrical insulationproperties, whilst being of relatively low thickness, and thus allowingcompact cable constructions.

In an alternative embodiment, the insulating tape may be a polyesterfilm, such as Mylar (trademark), Melenex (trademark), Hostaphan(trademark) or Milene (trademark). Such films include a polyester basefilm with a heat sealable thermoplastic polyester resin coating thereonwhich may be fused at relatively low temperatures e.g. 200°-210° C. Theinsulation is formed as before by tape-wrapping the conductor andheating to adhere the tape layers.

Typically, the cable comprises 2 to 12, typically 6, conductors. Theconductors may comprise twisted pairs.

The parallel conductors are held together, as shown in FIG. 2, bybraiding 10, comprising at least one filament interwoven between theinsulated conductors insulated 1, 2, 3, 4, 5, and 6. This allows theflat cable to be bent in any direction, including the direction parallelto the plane of the cable, which imparts good flexibility. It alsoallows the cable to be bent into a pre-configured harness configuration.Usually, the braiding will comprise a plurality of filaments, forexample 2 to 25 filaments, preferably 6 to 16 filaments. Typicalbraiding machines apply 13 filaments.

The filaments are preferably formed of an electrically insulatingmaterial. For some applications, the braiding is formed of a plasticsmaterial which becomes bonded to the electrically insulating coveringaround the conductors upon the application of heat. In particular, thebraiding may be formed of the same or similar material to that used forthe electrical insulation (e.g. the outer layer of tape-wrappedelectrical insulation) around the conductors. Thus, when the insulatingcovering around the conductors is formed of a material comprising PTFEand TFE/PPVE copolymer as disclosed in the above mentioned internationalpatent application, then the braiding is formed of the same or similaradherable material. The material may be bonded to itself by heating to asintering temperature.

In order to provide a braiding material of high tensile strength, anexpanded porous material of the type disclosed in the internationalpatent application PCT/GB91/00661 may be used. Such porous expandedmaterials may have a matrix tensile strength of up to 3515 kg/cm²(50,000 pounds per square inch), and are preferably drawn down to formflat fibres. The matrix tensile strength is defined as the tensilestrength divided by the porosity of the expanded porous material.

Analogously, if a polyester insulating material is used on theconductors, the braiding is preferably formed of the same material or asimilar material which can be adhered thereto under the effect of heat.

In this way, the flat cable may be bent into the desiredthree-dimensional configuration, and then heat-set by heating to asintering or fusing temperature.

The braiding filaments are preferably in the form of slit tapes of width1 to 4 mm, and thickness 5 to 20 microns.

Alternatively, the braiding filaments may be in the form of fibres orround monofilaments (such as nylon or polyester).

Preferably, the filaments are interwoven by passing a filament over oneconductor and under the adjacent conductor. Alternatively, groups ofconductors, for example 2 to 4 conductors twisted together, may beinterconnected by passing the filament over one group and under theadjacent group.

Thus, the present invention allows the production of a flat cable whichcan be bent in any direction in three dimensions so as to allow thepre-fabrication of a routed wiring harness. The harness configurationmay then be heat-set. The flat cable construction allows both flat andround terminal connectors to be used. The flat cable is simply rolled upin a transverse direction if a circular connector terminal is to beattached. A reduced thickness of insulation may be used, leading toincreased signal density and reduced cable weight.

PREFERRED EMBODIMENT

An embodiment of the present invention will now be described by way ofexample only in conjunction with the drawing wherein:

FIG. 2 shows a flat cable comprising six parallel insulated conductors 1to 6 which lie side-by-side in a single plane, and which are heldtogether by thirteen strips of braiding 10 interwoven between theconductors.

Each filament of braiding is woven over one conductor and under theadjacent conductor.

Each conductor has an electrically insulating covering 30 around it, asshown in FIG. 1. Typically, the insulated covering comprises a firstlayer of a spirally wound porous expanded tape; a second layer of anon-porous spirally wound tape; and a third outer layer of a porousexpanded spirally wound tape. The tapes are wound in overlapping (andpossibly counter-rotatory) overlapping turns. The porous and non-poroustapes are typically formed of a composite material as disclosed inPCT/GB91/00661 (W091/17551). The non porous material typically comprisesan intimate admixture of 5 to 40 wt.% of a thermoplastic copolymer oftetrafluoroethylene and perfluoro(propylvinylether) and 60 to 95 wt.% ofcoagulated dispersion type polytetrafluoroethylene, the compositematerial having been extruded and calendered to form a tape. The porousexpanded composite material typically is formed of an intimate admixtureof 50 to 90 wt.% of a thermoplastic copolymer of tetrafluoroethylene andperfluoro(propylvinylether) and 90 to 5 wt.% of polytetrafluoroethylene-Usually, the porous expanded material comprises 50 to 95 wt.% ofcopolymer.

The filaments of braiding are formed of the same porous expanded tapeand have a width 2 mm and thickness 12 microns. The matrix tensilestrength is 5624 to 7030 kg/cm² (80 to 100,000 pounds per square inch).The material is typically formed as Example 3 of W091/17551. The flatcable as shown in FIG. 1 may then be bent to shape in any direction. Dueto the fact that the conductors are held together by braiding, theconductors may move along side each other, thereby enabling the cable tobe bent in a direction parallel to the plane of the flat cable.

The cable has been formed into the desired configuration, it may beheat-set by sintering, typically by heat treating in air at 350° C. forabout 1 minute.

In an alternative embodiment six twisted pairs of insulated conductorsare braided into a flat cable. The conductor insulation is aheat-sealable polyester film and the braiding is formed of the samepolyester material, which is heat-settable at about 200°-210° C.

Other heat-settable insulating materials may be used for the insulationand braiding, and the present invention is not limited to any particularmaterial.

We claim:
 1. A flat cable which comprises a series of parallellongitudinally extending electrical conductors arranged side-by-side,each conductor having an electrically insulating covering, the parallelconductors being held together by braiding comprising at least onefilament interwoven between the conductors, wherein the braiding isthermoplastic and is heat-bonded to the insulating covering on theconductors.
 2. A cable according to claim 1, wherein the electricallyinsulating covering is in the form of spirally wound overlapping tape.3. A cable according to claim 2 where the insulating tape comprises twotapes; one being a polytetrafluoroethylene tape and the other being atape of a copolymer of tetrafluoroethylene and perfluoro(propylvinylether).
 4. A cable according to claim 3 wherein thepolytetrafluoroethylene tape is porous and the copolymer tape isnon-porous.
 5. A cable according to claim 1 wherein the braidingfilament is in the form of a tape.
 6. A cable according to claim 1wherein the braiding filament is in the form of a fibre or monofilament.7. A cable according to claim 6 wherein the braiding filament is in theform of a polytetrafluoroethylene fiber of substantially roundcross-section which has been formed from a continuous sheet ofpolytetrafluoroethylene which has been helically rolled and adhered toitself.
 8. A cable according to claim 1 comprising a plurality ofconductors and a plurality of braiding filaments, each filament passingover one conductor and under an adjacent conductor.